Railgun accelerators have met with limited success in accelerating projectiles from 1 gram to about 1 kilogram to velocities of about 7 km/s. Referring to FIGS. 1a and 1d, a railgun accelerator 10 having a pair of parallel spaced apart conducting rails 1a, 1b accelerates a projectile 2 along the rails 1 by establishing a high current plasma arc or armature 3 between the rails 1a, 1b and behind the projectile 2. The rails 1a, 1b are spaced apart by insulating wall spacers (or insulators) 11 which, together with rails 1a, 1b define the railgun barrel.
Under ideal conditions, there is only one current conduction path from rail 1a to rail 1b and it is located immediately behind the projectile 2. The magnetic fields from the currents in the rails 1a, 1b couple with the current in the armature to cause a Lorentz force on the plasma arc 3, which then results in a hydrodynamic acceleration pressure on the projectile 2.
In reality, arc growth and separation are aggravated by barrel-wall (or rail) ablation 4 as illustrated in FIG. 1b. Referring also to FIG. 1c, while the projectile 2 and the plasma arc 3 are being accelerated down the rails 1a, 1b gradual erosion of the railgun rails 1a, 1b and insulating wall spacers 11 causes a secondary arc, or restrike 5, to form in the debris left behind by the first armature 3.
In addition to causing the rail ablation 4, the high current plasma arc 3 causes ablation of the insulators 11. In particular, ablation of the insulating wall spacers 11 is much greater than the rail ablation and it introduces undesirable debris into the plasma arc 3 which increases the arc drag force. Hence, a large fraction of the driving force is required to move the arc 3, thus reducing the propulsive force that is available to move the projectile 2. In addition, since the rails 1a, 1b and the insulating wall spacers 11 are subject to erosion, they have to be replaced frequently.
The secondary arc 5 may form behind the neutral ablation products 4 of the first armature 3 or it may form further towards the breech of the railgun where the rail-to-rail voltage is higher and the gas pressure is lower. In either situation, the secondary arc 5 is undesirable because it reduces the propulsive capability of the railgun, thereby limiting the railgun operating velocity.
Specifically, the secondary arc 5 shunts current away from the primary, propulsive, plasma arc 3 employed to propel the projectile 2. The projectile acceleration force, F, diminishes with the primary current flowing in the propulsive plasma 3, I: where F=L'I.sup.2 /2 and L' is the inductance gradient of the rail pair. Hence, the propulsive force rapidly decreases as the shunt current grows.
Efforts have been made to accelerate projectiles at velocities greater than 8 to 9 km/s. However, as the velocities increase, the problem of restrike becomes more prevalent and high velocities are difficult to obtain.
A railgun projectile, used in conjunction with a railgun barrel having no insulating wall spacers is discussed in Evaluation of CAPEL, A Novel Railgun Concept, Australia Department of Defence, Defence Science and Technology Organization Materials Research Laboratories, Melbournem, Victoria, Report MRL-R-1018, (September, 1986). The railgun projectile has an internal, oval shaped cavity which completely confines a plasma armature within the projectile as the projectile is accelerated along the rails. In this case, the railgun barrel that is used in conjunction with the confined armature projectile design does not have insulating wall spacers. Rather, the walls of the projectile itself serve to confine the hot plasma. The disadvantage of this approach is that the plasma pressure tends to destroy the confining projectile as noted in the report. Hence, Applicant's invention uses the barrel to contain the plasma pressure thereby resulting in plasma contact with insulating rail spacers. By the plasma armature making direct contact with the insulating spacers, the plasma arc severely damages the insulating spacers in the region exposed to the plasma.
The Australian reference does not address the problem of railgun rail erosion caused by the plasma armature directly contacting the rails as the projectile is accelerated. In addition, the projectile design does not reduce or eliminate restrike.
U.S. Patent application, No. 07/341,019, filed Aug. 28, 1989, entitled "Prevention of Breakdown/Restrike Behind Railgun Projectiles," now U.S. Pat. No. 5,142,962 issued Sep. 1, 1992 is herein incorporated by reference. The Prevention of Breakdown/Restrike application is directed to a method-apparatus for preventing secondary voltage breakdown behind a railgun projectile while it is being accelerated by a plasma arc prior to launch. The Prevention of Breakdown application provides that restrike can be eliminated or reduced by configuring the projectile to have a cavity or a shielding skirt at its rear end and/or by fabricating the projectile out of a material which releases a breakdown inhibiting gas as the projectile is accelerated. In one embodiment of the invention the projectile is configured with a V-shaped arc extinguishing trailing skirt at the rear of the projectile. The arc extinguishing skirt shields the railgun rails from the plasma arc and interrupts the current flow to reduce ablation of the rails.